Turbulence as a Mechanism for Orographic Precipitation Enhancement

Abstract

This study examines the dynamical and microphysical mechanisms that enhance precipitation during the passage of winter midlatitude systems over mountain ranges. The study uses data obtained over the Oregon Cascade Mountains during the Improvement of Microphysical Parameterization through Observational Verification Experiment 2 (IMPROVE-2; November?December 2001) and over the Alps in the Mesoscale Alpine Program (MAP; September?November 1999). Polarimetric scanning and vertically pointing S-band Doppler radar data suggest that turbulence contributed to the orographic enhancement of the precipitation associated with fronts passing over the mountain barriers. Cells of strong upward air motion (>2 m s?1) occurred in a layer just above the melting layer while the frontal precipitation systems passed over the mountain ranges. Upstream flow appeared to be generally stable except for some weak conditional instability at low levels in the two IMPROVE-2 cases. The cells occurred in a layer of strong shear at the top of a low-level layer of apparently retarded or blocked flow (shown by Doppler radial velocity data). The shear apparently provided a favorable environment for the turbulent cells to develop. The updraft cells appeared at the times and locations where the shear was strongest (>?10 m s?1 km?1). The Richardson number was slightly less than 0.25 at the level where the cells were observed, suggesting shear-generated turbulence could have been the origin of the updraft cells. Another possibility is that the rough mountainous lower boundary could have triggered buoyancy oscillations within the stable, sheared flow. The existence of turbulent cells made possible a precipitation growth mechanism that would not have been present in a laminar upslope flow. The turbulent cells appeared to facilitate the rapid growth and fallout of condensate generated over the lower windward slopes of the mountains. In a laminar flow over terrain, upward motions would be unlikely to produce liquid water contents adequate to increase the density (and hence the fall speed) of precipitating ice particles by riming. The turbulent updraft cells apparently create pockets of higher values of liquid water content embedded in the widespread frontal cloud system, and snow particles falling from the parent cloud systems can then rapidly rime within the cells and fall out. Observations by polarimetric radar and direct aircraft sampling indicate the occurrence of rimed aggregate snowflakes and/or graupel in the turbulent layer. Inasmuch as the shear layer is the consequence of retardation or blocking of the low-level cross-barrier flow, and the turbulence is a response to the shear, the shear-induced cellularity is an indirect response of the flow to the topography. The turbulence embodied in this orographically induced cellularity allows a quick response of the precipitation fallout to the orography since aggregation and riming of ice particles in the turbulent layer produce heavier, more rapidly falling precipitation particles. Without the turbulent cells, condensate would more likely be advected farther up and perhaps even over the mountain range. Small-scale cellularity has traditionally been associated with the release of buoyant instability by the upslope flow. Our results suggest that cellularity may be achieved even if buoyant instability is weak or nonexistent, so that even a stable flow has the capacity to form cells that will enhance the precipitation fallout over the windward slopes.